Replacement of the cathode ray tube with a flat panel screen requires a display technology which simultaneously makes it possible to achieve a high resolution, i.e. more than 1000 lines, a high brightness ( greater than 200 cd/m2), a high contrast ( greater than 100:1), a high frame rate ( greater than 60 Hz), an adequate color representation ( greater than 16 million), a large image format ( greater than 40 cm), a low power consumption and a wide viewing angle, at low production costs. At present, there is no technology which fully satisfies all these features simultaneously.
Many manufacturers have developed screens which are based on nematic liquid crystals and have been used in recent years in the field of notebook PCs, Personal Digital Assistants, desktop monitors etc. Use is made here of the technologies STN (supertwisted nematics), AM-TN (active matrix-twisted nematics) AM-IPS (active matrix-in plane switching) and AM-MVA (active matrix-multidomain vertically aligned), which are described in the relevant literature (see, for example, T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references cited therein; SID Symposium 1997, ISSN-0097-966X, and the references cited therein). Furthermore, mention should be made of the technologies PDP (plasma display panel), PALC (plasma addressed liquid crystal), ELD (electroluminescent display), FED (field emission display) etc., which are also explained in the above-cited SID report.
Clark and Lagerwall (U.S. Pat. No. 4,367,924) have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in opto-electrical switching or display elements which have response times which are faster by a factor of up to 1000 compared with conventional TN (xe2x80x9ctwisted nematicxe2x80x9d) cells (see, for example, EP-A 0 032 362). Owing to this and other favorable properties, for example the possibility of bistable switching and the fact that the contrast is virtually independent of the viewing angle, FLCs are basically suitable for areas of application such as computer displays and TV sets, as shown by a monitor marketed in Japan by Canon since May 1995.
The use of FLCs in electro-optical or fully optical components requires either compounds which form tilted or orthogonal smectic phases and are themselves optically active, or the induction of ferroelectric smectic phases by doping corn-pounds which, although forming such smectic phases, are not themselves optically active, with optically active compounds. The desired phase should be stable over the broadest possible temperature range to ensure that the display has a broad operating range. In particular, the contrast obtainable should be as high as possible over the entire operating range.
In so-called active matrix technology (AMLCD), a nonstructured substrate is usually combined with an active matrix substrate. An electrically non-linear element, for example a thin-film transistor, is integrated into each pixel of the active matrix substrate. The nonlinear elements can also be diodes, metal-insulator-metal and similar elements, which are advantageously produced by thin-film processes and are described in the relevant literature (see, for example, T. Tsukuda, TTT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references cited therein).
Active matrix LCDs are usually operated with nematic liquid crystals in TN (twisted nematics), ECB (electrically controlled birefringence), VA (vertically aligned) or IPS (in-plane switching) mode. In each case, the active matrix generates an electric field of individual strength on each pixel, producing a change in alignment and thus a change in birefringence, which is in turn visible in polarized light. A severe disadvantage of this process is the poor video capability, i.e. excessively slow response times, of nematic liquid crystals.
For this and other reasons, liquid crystal displays based on a combination of ferroelectric liquid crystal materials and active matrix elements have been proposed, for example in WO 97/12355, Ferroelectrics 1996, 179, 141-152, or W. J. A. M. Hartmann (IEEE Trans. Electron. Devices 1989, 36 (9;Pt. 1), 1895-9, and dissertation, Eindhoven, The Netherlands, 1990).
While Hartmann utilizes the charge-controlled bistability to display a virtually continuous gray scale, Nito et al. have suggested a monostable FLC geometry (Journal of the SID, 1/2, 1993, pages 163-169) in which the FLC material is aligned by means of relatively high voltages such that only a single stable position results from which a number of intermediate states are generated when an electric field is applied via a thin-film transistor. These intermediate states correspond to a number of different brightness values (gray shades) when the cell geometry is matched between crossed polarizers.
The disdavantage of the paper by Nito et al. is the occurrence of a streaky texture which limits contrast and brightness of this cell (see FIG. 8 of the abovementioned citation). Furthermore, this method produces switching only in an angle range of up to a maximum of once the tilt angle, which is about 22xc2x0 in the case of the material used by Nito et al. (cf. p. 165, FIG. 6) and thus produces a maximum transmission of only 50% of the transmission of two parallel polarizers. Terada et al. have suggested a monostable FLC configuration (Applied Physics Conference, Mar. 28, 1999, Tokyo, Japan; Abstract No. 28p-V-8). However, these displays are not yet suitable for practical use over a relatively large temperature range.
The object of the present invention is to provide an active matrix liquid crystal display comprising a chiral smectic liquid crystal mixture where the liquid crystal mixture makes it possible to achieve a very high maximum transmission and a very high contrast and a constant threshold voltage over a broad temperature range.
In particular, a ferroelectric active matrix liquid crystal display comprising a ferrolelectric liquid crystal mixture is to be provided where the liquid crystal mixture assumes a monostable position, but without forming a streaky texture, is temperature-stable and makes it possible to achieve a very high maximum transmission and a very high contrast and a constant threshold voltage over a broad temperature range.
This object is achieved according to the invention by a chiral smectic active matrix display comprising a liquid crystal layer having the phase sequence Ixe2x80x94N*xe2x80x94Sc*, a tilt angle which is virtually constant over a broad temperature range and a virtually constant deviation of the monostable position (i.e. the position in which the transmission of an FLC display arranged between two crossed polarizers is at a mininum) from the rubbing direction. The invention accordingly provides an active matrix display comprising a chiral smectic liquid crystal mixture where the liquid crystal mixture is characterized by the phase sequence Ixe2x80x94N*xe2x80x94SmC*, a spontaneous polarization in the operating temperature range of  less than 40 nC/cm2 and a pitch of  greater than 10 xcexcm at at least one temperature in the nematic or cholesteric phase and comprises at least one compound each from at least two of the substance classes (A), (B) and (C) and from 0.1 to 50% by weight, based on the liquid crystal mixture, of one or more compounds from substance class (D) xe2x80x83R20xe2x80x94M18xe2x80x94(xe2x80x94A14xe2x80x94M14)a(xe2x80x94A15xe2x80x94M15)bxe2x80x94(M16xe2x80x94A16)cxe2x80x94(M17xe2x80x94A17)dxe2x80x94M19xe2x80x94R21xe2x80x83xe2x80x83(D)
where:
R1, R2, R3, R4, R5, R6 are each, independently of one another, hydrogen or a straight-chain or branched alkyl or alkenyl radical (with or without asymmetric carbon atoms) having 2 to 18 carbon atoms, where one or two nonterminal, nonadjacent xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94Oxe2x80x94 and/or one xe2x80x94CH2xe2x80x94 group may be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94 or xe2x80x94Si(CH3)2xe2x80x94 and one or more H atoms may be replaced by F with the provisos that heteroatoms cannot be adjacent and that in each case only one of R1, R2 or R3, R4 or R5, R6, respectively, can be hydrogen;
Y1, Y2, Y3, Y4, Y5, Y6 are each, independently of one another, xe2x80x94Oxe2x80x94, xe2x80x94OC(xe2x95x90)xe2x80x94, xe2x80x94C(xe2x95x90O)Oxe2x80x94, xe2x80x94OC(xe2x95x90O)Oxe2x80x94 or a single bond;
Z1 is xe2x80x94OC(xe2x95x90O)xe2x80x94 or xe2x80x94C(xe2x95x90O)Oxe2x80x94, 
is phenylene-1,4-diyl, unsubstituted, monosubstituted, disubstituted or trisubstituted by F, pyrimidine-2,5-diyl, unsubstituted or monosubstituted by F, pyridine-2,5-diyl, unsubstituted or monosubstituted by F, pyridazine-2,5-diyl, unsubstituted or monosubstituted by F, 
is phenylene-1,4-diyl, unsubstituted, monosubstituted, disubstituted or trisubstituted by F, pyrimidine-2,5-diyl, unsubstituted or monosubstituted by F, pyridine-2,5-diyl, unsubstituted or monosubstituted by F, pyridazine-2,5-diyl, unsubstituted or monosubstituted by F
with the proviso that one of the rings A, B is one of the nitrogen heterocycles mentioned, 
are each phenylene-1,4-diyl, independently of one another unsubstituted, mono substituted, disubstituted or trisubstituted by F
with the proviso that at least one of the rings C, D, E is fluorophenylene-1,4-diyl or ortho-difluorophenylene-1,4-diyl, 
are each, independently of one another, phenylene-1,4-diyl, unsubstituted, monosubstituted, disubstituted or trisubstituted by F, pyrimidine-2,5-diyl, unsubstituted or monosubstituted by F, pyridine-2,5-diyl, unsubstituted or monosubstituted by F, pyridazine-2,5-diyl, unsubstituted or monosubstituted by F
with the proviso that one of the rings F, G, K is one of the nitrogen heterocycles mentioned,
R20, R21 are each, independently of one another,
a) hydrogen or an alkyl, alkenyl, alkyloxy or alkenyloxy radical having 2 to 12 carbon atoms, where one or two nonadjacent xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94(Oxe2x95x90)Cxe2x80x94Oxe2x80x94, xe2x80x94Si(CH3)2xe2x80x94 or cyclopropane-1,2-diyl and one or more H atoms may be replaced by F or
b) a moiety having at least one asymmetric carbon atom which is either part of an alkyl group having 3 to 16 carbon atoms, where 1 to 4 nonadjacent xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94OC(xe2x95x90O)xe2x80x94 or xe2x80x94(Oxe2x95x90)Cxe2x80x94Oxe2x80x94 and one of the substituents of the asymmetric carbon atom is xe2x80x94CH3, xe2x80x94CF3, xe2x80x94OCH3, xe2x80x94CH3, Cl, F, CN or xe2x80x94OCF3, or part of a 3- to 7-membered heterocycle, where one or two nonadjacent xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94Oxe2x80x94 or one xe2x80x94CH2xe2x80x94 group may be replaced by xe2x80x94OC(xe2x95x90O)xe2x80x94 or xe2x80x94(Oxe2x95x90)Cxe2x80x94Oxe2x80x94,
with the provisos that the moiety as defined in b) having at least one asymmetric carbon atom is present in at least one of R20, R21 and M18 or M19 is a single bond if the moiety having the asymmetric carbon atom is part of an alkyl chain,
A14, A15, A16, A17 are each, independently of one another, 1,4-phenylene, unsubstituted, monosubstituted or disubstituted by F or Cl, 1,3-phenylene, unsubstituted, monosubstituted or disubstituted by F or Cl, cyclohexane-1,4-diyl, unsubstituted or monosubstituted by F or CN, cyclohex-1-ene-1,4-diyl, 1-5fluorocyclohex-1-ene-1,4-diyl, cyclohex-2-ene-1,4-diyl, 2-oxocyclohexane-1,4-diyl, 2-cyclohexen-1-one-3,6-diyl, 1-alkyl-1-sila-cyclohexane-1,4-diyl, bicyclo[2.2.2]octane-1,4-diyl, spiro[4.5]decane-2,8-diyl, spiro[5.5]undecane-3,9-diyl, indane-2,5-diyl, naphthalene-2,6-diyl, unsubstituted, monosubstituted or disubstituted by F or CN, pyrimidine-2,5-diyl, unsubstituted or monosubstituted by F, pyridine-2,5-diyl, unsubstituted or monosubstituted by F, pyrazine-2,5-diyl, unsubstituted or monosubstituted by F, pyridazine-3,6-diyl, quinoline-2,6-diyl, quinoline-3,7-diyl, isoquinoline-3,7-diyl, quinazoline-2,6-diyl, quinoxaline-2,6-diyl, 1,3-dioxane-2,5-diyl, thiophene-2,4-diyl, thiophene-2,5-diyl, 1,3-thiazole-2,4-diyl, 1,3-thiazole-2,5-diyl, isoxazole-3,5-diyl, benzthiazole-2,6-diyl, unsubstituted, monosubstituted or polysubstituted by F, benzthiazole-2,5-diyl, unsubstituted, monosubstituted or polysubstituted by F, 1,3,4-thiadiazole-2,5-diyl, piperidine-1,4-diyl or piperazine-1,4-diyl,
M14, M15, M16, M17 are each, independently of one another, a single bond, xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94(Oxe2x95x90)Cxe2x80x94Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94(CH2)4xe2x80x94 or xe2x80x94Cxe2x95x90Cxe2x80x94,
M18, M19 are each, independently of one another, xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94(Oxe2x95x90)Cxe2x80x94Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94 or a single bond,
a, b, c, d are each, independently of one another, zero or 1 with the proviso 1xe2x89xa6{a+b+c+d}xe2x89xa63 and the understanding that (xe2x80x94Axxe2x80x94Mxxe2x80x94) is a single bond when the corresponding index is zero.
Component (D) is preferably present in an amount from 0 to 30% by weight, in particular from 0.05 to 20% by weight.
Here and likewise hereinbelow, it will be understood that bivalent radicals were designated in the xe2x80x9cfree statexe2x80x9d. This designation is essential for the characterization of the compounds, although strictly in accordance with IUPAC rules, other designations of the bivalent radicals forming part of the entire Markush formula-meaning incorporation both as image and as mirror image would be possible.
In particular, the object is achieved according to the invention by using a chiral smectic liquid crystal mixture in which there is a monostable position of the director (longitudinal axis) and the deviation of this monostable position from the rubbing direction in the industrially relevant temperature range from 10 to 50xc2x0 C. is less than 10 degrees, preferably less than 5 degrees, more preferably less than 4 degrees and particularly preferably less than 3 degrees.
The term xe2x80x9cindependently of one anotherxe2x80x9d means that each of the stated radicals may be selected from the stated meanings independently of the selection of the other radicals. The radicals can thus be identical or different.
The advantageous phase sequence Ixe2x80x94N*xe2x80x94Sc* (also termed Ixe2x80x94Nxe2x80x94C) in the context of the present invention applies even if a narrow SmA phase range exists between the N* phase and the Sc* phase, with said range not exceeding a temperature range of 2K.
Futhermore it may be advantageous for the LCD cell to have an asymmetrical structure, i.e. the top surface and the bottom surface of the cell differ in at least one feature apart form the active matrix itself. This is in particular the case:
when using unsymmetrical or unsymmetrically treated alignment layers (for example in the case of antiparallel rubbing)
when one of the two alignment layers is omitted
when the step of rubbing one of the two alignment layers is omitted or changed
when an unsymmetrical layer structure is introduced, for example by additional insulation layers having different properties on their top and bottom surfaces
with all measures which finally result in exposure of the liquid crystal domain to an environment which is unsymmetrical in relation to a symmetry plane parallel to the electrode surfaces.
Expressly included is the advantageous use of the novel materials and mixtures for active matrix displays, antiferroelectric displays and smectic displays, the term xe2x80x9cdisplayxe2x80x9d being intended to mean any type of optical display or switching device regardless of its size, structure, light guidance, addressing and use.
In particular, the term xe2x80x9cactive matrix displayxe2x80x9d as used herein includes an LCD in which one of the two substrates is replaced by the rear side of an IC chip (IC=integrated circuit) as described, for example, in D. M. Walba, Science 270, 250-251 (1995) or http://www.displaytech.com.
In particular, the object is achieved by a chiral smectic active matrix display comprising a liquid crystal layer in the form of a monostable liquid crystal domain having a tilt angle which is virtually constant over a broad temperature range.
The terms xe2x80x9cdomainxe2x80x9d or xe2x80x9cmonodomainxe2x80x9d as used herein mean a range of essentially constant director configuration which distinguishes the monostable display from the bistable conventional SSFLC display. While the SSFLC display features two stable director configurations which are optically very different, the active matrix display of the invention has only one domain whose director changes continuously with the voltage and returns to the same stable state when the voltage is switched off. The presence of a possible fine structure of this domain is irrelevant for the display of the invention provided that its elements have essentially the same optical properties as the domain itself and thus a high contrast can be achieved.
The processes for preparing the components of the liquid crystal mixtures of the active matrix displays according to the invention are known in principle, as is the preparation of liquid crystal mixtures from the individual components (see, for example, DE-A 198 57 352).
It has been found in accordance with the invention that active matrix displays in which the ferroelectric smectic phase is stable over a broad temperature range are obtainable by using the liquid crystal mixtures comprising at least one compound each from at least two of the substance classes (A), (B) and (C) and one or more compounds from substance class (D). Furthermore, the deviation of the monostable position from the rubbing direction is small in terms of magnitude and virtually constant over a broad temperature range. As a result, maximum contrast is achieved in the entire temperature range at a fixed polarizer position.
The minimum requirements for the mixture are thus at least one compound from the substance classes (A), (B), (D) or (A), (C), (D) or (B), (C), (D).
Preferred compounds of the individual substance classes are listed hereinbelow, the radicals being as defined above unless indicated otherwise.
Preferred compounds of the substance class (A) correspond to the formulae 
Preferred compounds of the substance class (B) correspond to the formulae 
where Y3 and Y4 are each, independently of one another, a single bond or xe2x80x94Oxe2x80x94.
Preferred compounds of the substance class (C) correspond to the formulae 
where the ring F is pyrimidine-2,5-diyl or pyridine-2,5-diyl or 2-fluoropyridine-3,6-diyl, 
where the ring G is pyrimidine-2,5-diyl or pyridine-2,5-diyl or 2-fluoropyridine-3,6-diyl, 
where the ring K is pyrimidine-2,5-diyl or pyridine-2,5-diyl and Y5 is in each case a single bond or xe2x80x94Oxe2x80x94.
Preferred compounds of the substance class (D) are those in which said moiety having said at least one asymmetric carbon atom-which is termed R* here and hereinafterxe2x80x94in R20 or R21, respectively, contains at least one of the structural elements
a) xe2x80x94C*H(F)xe2x80x94
b) xe2x80x94C*H(F)xe2x80x94C*H(F)xe2x80x94
c) xe2x80x94C*H(Cl)xe2x80x94
d) xe2x80x94C*H(CH3)xe2x80x94
e) xe2x80x94C*H(CF3)xe2x80x94
f) -oxirane-2,3-diyl-
where A 14, A 15, A16, A17 are preferably each, independently of one another, phenylene-1,4-diyl, 2-fluorophenylene-1,4-diyl, 2,3-difluorophenylene-1,4-diyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl, 2-fluoropyridine-3,6-diyl, cyclohexane-1,4-diyl, 1-cyanocyclohexane-1,4-diyl. It is also preferred that A14 is cyclohexane-1,4-diyl when R20 is hydrogen.
Particular preference is given to the compounds 
in which the sum of the carbon atoms in R1 and R2 is from 14 to 22; in particular R1 and R2 contain at least 6 carbon atoms each; 
in which Y3 is xe2x80x94Oxe2x80x94, Y4 is a single bond and the sum of the carbon atoms in R3 and R4 is from 10 to 20, R3 and R4 preferably containing at least 4 carbon atoms each.
Particularly preferred compounds of the formulae C1, C2, C3 are those in which R5 is an alkyl group having 4 to 10 carbon atoms and R6 is an alkyl group having 5 to 12 carbon atoms.
Particularly preferred compounds of the substance class (D) are those in which one of A14, A15, A16, A17 is a pyrimidine-2,5-diyl or pyridine-2,5-diyl radical and R* is an alkyl group, where one xe2x80x94CH2xe2x80x94 group is replaced by xe2x80x94OC(xe2x95x90O)xe2x80x94 or xe2x80x94C(xe2x80x94O)Oxe2x80x94, at least one other (nonadjacent) xe2x80x94CH2xe2x80x94 group is replaced by xe2x80x94Oxe2x80x94 and the structural element xe2x80x94C*H(CH3)xe2x80x94 is present once or twice.
Other particularly preferred compounds of the substance class (D) are those in which one of A14, A15, A16, A17 is a pyrimidine-2,5-diyl or pyridine-2,5-diyl radical and R* is an alkyl group in which the structural element -oxirane-2,3-diyl- is present once.
The liquid crystal mixture of the display according to the invention particularly preferably comprises, in total, from 0.1 to 15% by weight of one or more compounds from substance class (D); most preferably the mixture comprises, in total, from 0.2 to 12% by weight of one or more compounds from substance class (D), based on the entire mixture. Preference is given to a mixture which has a spontaneous polarization in the operating temperature range of  less than 20 nC/cm2. Moreover, preference is given to a display wherein the liquid crystal mixture comprises at least two compounds of the formula A2a 
in which the sum of the carbon atoms in R1 and R2 is from 14 to 22.
Furthermore, preference is given to a display wherein the liquid crystal mixture further comprises at least one compound from substance class (B), in which one of the rings C, D and E is fluorophenylene-1,4-diyl, Y3, Y4 are each, independently of one another, a single bond or xe2x80x94Oxe2x80x94 and the sum of the carbon atoms in R3 and R4 is from 10 to 20, and at least one compound from substance class (C), in which the nitrogen-containing ring is pyridine-2,5-diyl, R5 is an alkyl group having 4 to 10 carbon atoms and R6 is an alkyl group having 5 to 12 carbon atoms. The invention also relates to the liquid crystal mixtures described above.